This invention generally relates to the art of filament wound composite structures and, particularly, to a method of fabricating a filament wound vessel, such as a pressure vessel, chemical or water storage tanks, rocket motor cases or any tubular structure that requires end closures. Tubular structures may be cylindrical, rectangular, triangular, ellipsoidal, square or other polygonal shapes.
Various types of vessels, such as pressure vessels, rocket casings, and other tubular structures and the like, are fabricated with wall structures of filament wound composite materials. In other words, the thickness of the walls is built up by winding filaments associated with an appropriate resin, whereafter the assembly is cured to form what is called a xe2x80x9cfilament composite structurexe2x80x9d. The filaments may be of glass, graphite or like material. The thickness of the structural walls are built up by winding hoop or helical layers in a desired pattern. In some instances, an impermeable liner or bladder may be used inside the filament composite structure. In some instance, a core or foam layer or thickness may be sandwiched between filament wound layers. A structural inner skin or layer and a structural outer skin or layer may be separated by a hollow space that may be vacuumed to reduce heat transfer between the inner and outer skins. The core may be made of lightweight material such as a honeycomb core, closed foam, balsa wood or the like. Various fittings, such as a polar boss at a closed end of the vessel, may be integrated in the composite structure and held in place by the filament windings. A manway may be installed along the cylindrical surface of the structure with no openings at the domes except discharging faucets and the like.
Filament wound vessels of the character described above typically are fabricated by winding and building-up the composite wall structure on a mandrel. The mandrel and the resulting filament wound vessel typically has one or more dome-shaped closed ends. For instance, a filament wound vessel may have a generally cylindrical portion extending a major length of the vessel corresponding to a cylindrical mandrel about which the vessel is wound. The ends of the mandrel are generally ellipsoidal to form opposite dome-shaped closed ends which may or may not have various fittings, such as polar bosses. The cylindrical portion of the vessel is formed by hoop and longitudinal filament windings. The longitudinal windings may be wound using either helical or polar patterns and extend into the domes at each end of the cylindrical section. After the filament composite structure of the vessel is built-up on the mandrel, the entire assembly is cured and the vessel is removed from the mandrel.
Problems continue to plague fabrication processes as described above, particularly in removing the mandrel or mandrel tooling. Segmented metal mandrels have been used with some success. A segmented mandrel is broken down and removed through an opening in the vessel (i.e., from the inside-out). Unfortunately, segmented mandrels are extremely expensive, very labor intensive and sacrifice tolerance repeatability. Water soluble sand mandrels have been effective for large pressure vessels and rocket motor casings, but the binder in the sand is limited to low temperature cures. In addition, dimensional repeatability of sand surfaces can be a variable or requires extremely expensive tooling. Simply cutting the vessel and reattaching domes by bolts presents sealing problems, particularly when the vessel is used as a pressure vessel.
The present invention is directed to solving this myriad of problems by a simple fabrication process which not only affords easy removal of the mandrel tooling used in fabricating a pressure vessel, but the originally wound closed end of the vessel can easily be replaced by a different closed end which may be very difficult or impossible to fabricate by filament winding processes.
An object, therefore, of the invention is to provide a new and improved method of fabricating a filament wound container of the character described.
In the exemplary embodiment of the invention, the method includes the steps of providing a mandrel for winding a filament composite structure of a given thickness and having a closed end. The composite structure is filament wound on the mandrel, and the closed end is removed from the remainder of the composite structure. The mandrel is removed from the structure, and a replacement closed-end member is replaced on the structure at the location of the removed closed end. An overwrap thickness then is filament wound about at least a portion of the composite structure and the replaced closed-end member.
As disclosed herein, the mandrel and the container are generally cylindrical with dome-shaped closed ends. One of the dome-shaped ends is removed from the cylindrical remainder of the composite structure by cutting along a tangent line between the closed end and the cylindrical section of the structure.
The removed dome-shaped closed end may be replaced with a closed-end member of a different configuration, such as a relatively flat closed-end member. The removed dome-shaped closed end may be replaced with a closed-end member of a different material or with a member having multiple openings which would be difficult or impossible to fabricated by filament winding processes.
The invention contemplates that a joint section can be embedded in the composite structure, with the dome-shaped closed end being removed along the joint section. Seal means are provided on the joint section after the dome-shaped closed end is removed, for sealing with the replaced close-end member. For instance, circular grooves may be formed in a face of the joint section for receiving the seal means, such as O-ring seals.
Other objects, features and advantages of the invention will be apparent from the following detailed description taken in connection with the accompanying drawings.